When some structural materials are exposed to particular aggressive service environments under steady or cyclic stress, stress corrosion cracking ("SCC") can occur. It is desirable to monitor and assess the extent of damage to structural components due to SCC, for example, in a boiling water reactor ("BWR") which has been operating for a number of years to help predict its lifetime. Crack growth sensors are placed in the core of nuclear reactors as well as in piping flanges outside the core.
Coffin et al. U.S. Pat. No. 4,677,855 discloses a sensor for measuring growth of a preformed crack within a solid exposed to an aggressive environment during application of a load. The crack is defined as possessing a mouth and a tip. The "mouth of the crack" is defined as the point or line of action of load application. The "crack tip" is the leading edge of the crack. The "length" of the crack is defined as the distance from the mouth of the crack to the crack tip. The preformed crack within the solid is of a known length.
The sensor material must be electrically conductive. A current is passed through the solid to establish a voltage drop across the crack. This voltage is measured by at least two pairs of probes, the probes of each pair being positioned on opposite sides of the crack at equal, known distances from the crack mouth.
When a current is caused to flow through the sensor perpendicular to the crack, the potential difference between two points located on opposite sides of the crack will increase as the size of the crack increases. Measurement of the electric potential will provide information as to the instantaneous damage as well as the accumulated damage to the sensor in the form of crack growth.
The configuration of the sensor should permit an applied load of sufficient magnitude to provide a crack tip stress intensity factor that allows the crack to grow at an appropriate rate. A sensor having a double-cantilever beam ("DCB") geometry permits a load of sufficient magnitude to be conveniently applied. As shown in FIG. 1, a sensor 10 with DCB geometry has two parallel arms (beams) 12 and 14 joined at one end and separated at the other. A slot or deep notch 16 separates the arms. The base of this notch is referred to as the notch root 18. The preformed crack 20 is preferably located at the notch root. This configuration permits a number of measurements to be taken at various positions along the beams 12 and 14 since the effective crack length is extended along these beams. The stress intensity is a function of DCB cross section; crack length and applied load. Therefore, the long length of the sensor permits the threshold crack tip stress intensity to be obtained at low load levels.
Side grooves 34 placed within the sensor along the plane of the preformed crack determine the plane in which the crack grows. It is important to keep the fracture surfaces of the crack as planar as possible to avoid multiple cracking and bridging of the crack. Bridging can lead to a short circuit in the current flow and cause errors in the electric potential measurements.
Crack growth is preferably monitored by measuring a potential or voltage across pairs of probes disposed along beams 12 and 14, and using such measured voltages, as well as the initial parameters, to calculate a crack length. At least three pairs of probes 26a/26b, 28a/28b and 30a/30b are preferred for accurate measurement of the crack growth.
A pressure boundary 36 which serves as a junction box provides protection for the probe wires from the aggressive environment. Channel 38 provides access to channels (or holes) 40, both of which provide pathways for the conductive leads attached to the probe pairs and to conductive leads which preferably supply a d.c. potential to the sensor. The reversing direct current is supplied at points 32a and 32b and the effective initial length of the crack is indicated by dimension .alpha..sub.0.
For monitoring SCC in aggressive environments, it is conventional practice to apply a fixed displacement to the sensor, thereby causing the preformed crack to grow. A fixed displacement is applied by forcing a wedge 24 into the notch to expand the crack. The wedge must be made of electrically nonconductive material.
When the stress intensity factor is constant at the leading edge of the crack, the resulting rate of crack growth is expected to be constant. Although wedge-loaded DCB crack growth sensors have been successfully used, the stress intensity tends to decrease due to extended crack length and material creep. Thus, actively loaded sensors allowing remote control of the load can compensate for intrinsic load drops due to crack growth, creep and neutron-induced stress relaxation. In addition, load cycling or a change of load level can be performed. This increase in load application flexibility results in a substantial improvement to the sensor design. In essence, elastic energy which is stored in the beams of the DCB specimen is available to cause crack growth when a sufficient load is applied to spread apart the beams. As the crack grows, or as the specimen is heated, the elastic energy in the specimen decreases. In a stiff system, i.e, a sensor that is made of a material with a low compliance, the load drops of rapidly as the crack grows. With the resulting decrease in elastic energy, the rate of crack growth will generally slow down, and if a threshold stress intensity factor is not maintained, the crack growth will arrest completely. Since, without crack growth, no data for predicting crack growth can be generated, this is not a desirable condition. A method of applying a load having a constant stress intensity factor is needed to accommodate for the changing elasticity of the DCB specimen.